The invention relates to a vertical boat for holding a plurality of semiconductor wafers, which comprises two end members and a plurality of support members vertically mounted on the end members.
In an oxidation and/or a diffusion process, a wafer boat loaded with a plurality of semiconductor wafers is transferred into a diffusion furnace, and the wafers are subjected to a predetermined heat treatment.
Either a vertical boat or a horizontal boat is selected according to the type of the diffusion furnace.
A conventional vertical boat is shown in FIG. 1, which has two end plates (not shown) and a plurality of, for example four, vertical support bars 2 mounted on the end plates. A plurality of wafers 1 are held in horizontal or somewhat inclined manner on the vertical boat. A plurality of slits 2a are formed on the support members 2 at a predetermined interval for receiving semiconductor wafers 1.
In FIG. 1 each support bar 2 is exaggerated to be thick for easy understanding, however it is actually slender.
In the conventional vertical boat all support members 2 have, normally, the same configuration in cross section. The support member 2 is, for instance, a triangle bar or a rectangle bar. Each slit 2a formed on such a bar member has relatively a small support portion for supporting the wafer 1. The support portion has only a small extension in the circumferencial direction of the wafer 1. Two front support members are normally positioned at two ends of the diameter of the wafer 1 perpendicular to the wafer inserting direction X.
During heat treatments not only wafers 1 but the boat, regardless of its shape, is heated up to a high temperature. The larger the wafer 1 is, the more it is apt to be deflected by its own weight when heated up to a high temperature.
When the wafer is properly placed on the support members 2 of the conventional boat, the angle formed between the wafer inserting direction X and a line linking a front edge of the front support member to the gravity center of the wafer 1 is about 90 degrees, or at the most greater than 90 degrees by a few degrees.
In such a positioning, the gravity center of the wafer 1 is approximately located between the front edges of the two front support members 2 or its neighborhood so that the load of the wafers is biased on the two front support members. As a result, the two front support members share, for example, 70-90% of the weight of the wafers.
Therefore the larger the-wafers are, the more frequently slips are caused. Slips are caused by stress concentration especially at the portions engaging with the support members 2. Slip means crystal defect or crystal dislocation.
On the other hand, Japanese Utility Model laid open No. 62-128633 discloses a boat which comprises a plurality of circular arc plates fixed on support bars, wherein the loading stress of the large wafer can be somewhat relaxed. However, this boat is expensive because the circular arc plate must have a very accurate surface on which the wafer is to be placed.